The head of the European Commission, Jose Manuel Barroso, spoke of his "deep shock and horror" upon hearing of the massacre at Sandy Hook Elementary School, which he called a "terrible tragedy."

"Young lives full of hope have been destroyed," he said in a statement.

There were almost no non-fatal injuries, indicating that once targeted, there was little chance of escape, and that the gunman, believed to be 20-year-old Adam Lanza, was unusually accurate or methodical in his fire.

"The news is just awful. The thoughts and prayers of Canadians are with the students and families in CT affected by this senseless violence," Canadian Prime Minister Stephen Harper wrote on his Twitter account.

His foreign minister, John Baird, said Canadians "stand shoulder to shoulder with our American friends at this difficult time."

The child victims were reported to be aged between five and 10.

Pope Benedict XVI sent a letter of condolence to the community, which was read aloud at a vigil in Newtown Friday evening.

The pope "has asked me to convey his heartfelt grief and the assurance of his closeness in prayer to the victims and their families, and to all affected by the shocking event," Vatican Secretary of State Tarcisio Bertone said in the letter.

"In the aftermath of this senseless tragedy he asks God our Father to console all those who mourn and to sustain the entire community," the letter said.

British Prime Minister David Cameron said he "was shocked and deeply saddened to hear about today's horrific shooting."

"My thoughts are with the injured and those who have lost loved ones. It is heartbreaking to think of those who have had their children robbed from them at such a young age, when they had so much life ahead of them."

Britain's Queen Elizabeth II sent a message to President Barack Obama in which she said she was "deeply shocked and saddened" to hear of the shootings.

"The thoughts and prayers of everyone in the United Kingdom and throughout the Commonwealth are with the families and friends of those killed and with all those who have been affected by today's events."

French President Francois Hollande also extended his condolences in a message to Obama.

Even longtime foe Iran offered its condolences to the victims and families.

Foreign ministry spokesman Ramin Mehmanparast drew parallels between the school shooting and the "children and teenagers who fall victim to armed actions... inside Gaza, the U.S., Afghanistan, Pakistan, Iran or Syria," calling on Americans to mobilize against "warmongering and the massacre of innocent people anywhere."

Closer to home, Mexico's newly inaugurated president, Enrique Pena Nieto, also expressed his support to the U.S. after the deadly shooting.

"My solidarity with the American people and President @BarackObama after the tragedy this morning in Connecticut," he wrote on Twitter.

And Australian Prime Minister Julia Gillard said: "Australia grieves with America today following the mass shooting of primary school children and teachers in Connecticut.

"Like President Obama and his fellow Americans, our hearts too are broken.

"We share America's shock at this senseless and incomprehensible act of evil."

Philippine President Benigno Aquino on Saturday offered his support, expressing "deep admiration over the manner in which the American people have reached out to comfort the afflicted.

"We pray for healing, and that this heartbreak will never be visited on any community ever again," he said in a statement.